1. Field of the Invention
The present invention relates to a multilayer piezoelectric actuator device or another multilayer piezoelectric device and a method of producing the same, more particularly relates to a multilayer piezoelectric device and method of producing the same superior in moisture resistance.
2. Description of the Related Art
In recent years, along with the increasing precision of machine tools, industrial machines, measurement machines, and other precision machinery, there has been a strong demand for establishment of super-precision positioning technology offering a high reliability of operation and enabling high speed operation. Further, progress in micromachines has led to a strong demand for development of microactuators.
In the past, various microactuators have been known. For the following reasons, however, a multilayer piezoelectric actuator device has been viewed as promising as a multilayer piezoelectric viewed as promising as a multilayer piezoelectric device. That is, a multilayer piezoelectric actuator device is small in size, enables microdisplacement precision control, offers a fast response speed, generates a large force, and is high in energy conversion efficiency.
In general, a multilayer piezoelectric actuator comprises a plurality of piezoelectric layers and internal electrode layers alternately stacked.
In the past, the internal electrode layers in multilayer piezoelectric actuator devices, as shown in Japanese Unexamined Patent Publication (Kokai) No. 9-261978, have been generally comprised of inexpensive silver or a silver and palladium alloy.
Such a conventional multilayer actuator device, however, suffers from a tendency of the insulating property of the piezoelectric layers to fall under a high humidity environment and therefore has a problem in durability. This is believed to be because migration of the silver in the electrodes (ionization of silver and movement between electrodes) occurs under a highly humid environment. Depending on the application of the multilayer piezoelectric actuator device etc., sometimes the ends of the internal electrodes have to be exposed from the end faces of the device. In such a device, moisture resistance is particularly required.
Further, Japanese Patent No. 2826078 discloses a multilayer actuator device having internal electrode layers comprised of platinum, palladium, rhodium, silver-palladium, silver-platinum, and platinum-palladium. As shown in this publication, however, when using silver-palladium, silver-platinum, or another metal mainly comprised of silver as the internal electrode layers, the silver in the electrodes is liable to undesirably migrate in a highly humid environment due to the above reasons. Further, as shown in this publication, when using platinum, palladium, or rhodium alone or alloys of the same as the internal electrode layers, there is little shrinkage of the electrode layers compared with shrinkage of the piezoelectric layers due to sintering in the process of production of the device, so that the piezoelectric layers and internal electrode layers peel apart during firing, that is, delamination occurs. Due to this delamination, the reliability of the device against moisture deteriorates.
An object of the present invention is to provide a multilayer piezoelectric actuator device or other multilayer piezoelectric device free from deterioration of the insulation resistance of the piezoelectric layers, superior in moisture resistance, and having a high reliability and a method of producing the same.
To achieve the above object, the multilayer piezoelectric device according to the present invention comprises piezoelectric layers and internal electrode layers alternately stacked, the internal electrode layers containing gold as an essential ingredient.
Further, according to the inventor, the method of producing a multilayer piezoelectric device comprised of piezoelectric layers and internal electrode layers alternately stacked, comprises the steps of including gold in the internal electrode layers as an essential ingredient and firing the piezoelectric layers and internal electrode layers at a temperature of 1020 to 1220xc2x0 C., more preferably 1060 to 1200xc2x0 C.
Preferably, the content of the gold in the internal electrode layers is 20 to 90 wt %, more preferably 60 to 90 wt %, particularly preferably 60 to 80 wt %, with respect to the total weight of the metal ingredients in the internal electrode layers as 100 wt %.
Preferably the internal electrode layers contain gold as an essential ingredient and preferably contain at least one element from palladium, platinum, and rhodium, more preferably at least one element from palladium and platinum, particularly preferably palladium. The total content of the palladium, platinum, and rhodium in the internal electrode layers is paticularly preferably 10 to 80 wt %, more preferably 10 to 40 wt %, preferably 20 to 40 wt % with respect to the total weight of the metal ingredients in the internal electrode layers as 100 wt %. Particularly, the content of the palladium is preferably 20 to 40 wt %, more preferably 25 to 35 wt %.
Preferably, the thickness of the internal electrode layers is 1 to 3 xcexcm.
Preferably, the area of the piezoelectric layers which the internal electrode layers actually cover, that is, the coverage rate, is 50 to 99 percent, more preferably 70 to 90 percent. Note that the xe2x80x9ccoverage ratexe2x80x9d means the rate obtained by assuming a virtual predetermined area on the surface of an internal electrode layer, defining that predetermined area as 100 percent, and expressing the area of a piezoelectric layer which the internal electrode layer actually covers, that is, the coverage, as a percent.
If no holes etc. are formed in the internal electrode layer, the coverage rate is 100 percent.
Preferably, each piezoelectric layer comprises a lead-based piezoelectric material.
Preferably, the lead-based piezoelectric material contains a composition of the formula:
Pba[(xcex21,xcex22)xZryTiz]bO3
where xcex21 is at least one element selected from Zn, Cd, Mg, Ni, Co, Fe, Sc, Cr, Yb, Lu, In, Mn, and Sn and xcex22 is at least one element selected from Nb, Ta, W, Te, and Sb and
x, y, z, a, and b are in relations of 0 less than x less than 0.5,
0.1xe2x89xa6yxe2x89xa60.55,
0.2xe2x89xa6zxe2x89xa60.60,
x+y+z=1, and
0.97xe2x89xa6a/bxe2x89xa61.
Alternatively, the lead-based piezoelectric material contains a composition of the formula:
(Pb1xe2x88x92uxcex1u)a[(xcex21,xcex22)xZryTiz]bO3
where xcex1 is at least one element selected from Sr, Ba, Ca, and La, xcex21 is at least one element selected from Zn, Cd, Mg, Ni, Co, Fe, Sc, Cr, Yb, Lu, In, Mn, and Sn, and xcex22 is at least one element selected from Nb, Ta, W, Te, and Sb and
x, y, z, a, b, and u are in relations of
0 less than x less than 0.5,
0.1xe2x89xa6yxe2x89xa60.55,
0.2xe2x89xa6zxe2x89xa60.60,
x+y+z=1,
0.97xe2x89xa6a/bxe2x89xa61, and
0 less than uxe2x89xa60.15.
When using platinum, palladium, or rhodium alone or alloys of the same as the internal electrode layers, there is little shrinkage of the electrode layers compared with shrinkage of the piezoelectric layers due to sintering in the process of production of the device, so that the piezoelectric layers and internal electrode layers peel apart, that is, delamination occurs. Due to this delamination, the reliability of the device against moisture deteriorates. It has been considered to mix a low melting point metal and high melting point metal together to make the shrinkage behavior similar to that of the piezoelectric layers and thereby make delamination difficult. As the low melting point metal, silver or copper may be considered. In an internal electrode layer containing these metals, however, migration easily occurs under a high humidity environment.
In the multilayer piezoelectric device and method of producing the same according to the present invention, by designating gold as an essential ingredient and including palladium, platinum, and/or rhodium in the internal electrode layers, the adhesion with the piezoelectric layers can be maintained well and the moisture resistance can be improved.
In the present invention, if the thickness of the internal electrode layers is too small, the internal electrode layers tend to easily become discontinuous, while if the thickness is too large, delamination tends to easily occur due to the difference of shrinkage between the piezoelectric layers and internal electrode layers at the time of sintering. Further, if the thickness of the internal electrode layers is too large, in the case of an actuator device, displacement of the piezoelectric layers tends to be hampered, so the material is wasted. This leads to an increase in the manufacturing costs.
After firing, the internal electrode layer sometimes is formed with fine holes such as holes through which the solvent in the metal paste for forming the internal electrode layers escapes. Due to these fine holes etc., the area of a piezoelectric layer which an internal electrode layer actually covers, that is, the coverage, sometimes becomes smaller than the total area scheduled to be covered. As the internal electrode layers are fired, the fine holes combine to form large holes and the coverage falls. Further, the internal electrode layers also shrink due to exposure to high temperatures. Holes are made larger due to this.
In the present invention, by controlling the area of the piezoelectric layers which the internal electrode layers actually covers, that is, the coverage (coverage rate), to 50 to 99 percent, preferably 70 to 90 percent, sufficient displacement of the actuator device can be obtained and delamination can be suppressed. If the coverage rate is too small, sufficient displacement of the actuator device tends to be unobtainable, while if the coverage rate is too large, delamination tends to easily occur.
In the present invention, the material of the piezoelectric layers is not particularly limited, but is preferably comprised of a lead-based piezoelectric material of a firing temperature of a relatively low temperature of 1000 to 1250xc2x0 C. By configuring the invention in this way, the behavior of the piezoelectric layers at the time of firing becomes similar to the behavior of the internal electrode layers containing gold as an essential ingredient at the time of firing and the occurrence of delamination etc. can be suppressed.
In the present invention, the firing temperature is preferably 1020 to 1220xc2x0 C., more preferably 1060 to 1200xc2x0 C. If the firing temperature is too low, the sintering tends to become insufficient, while if too high, discontinuation of the electrodes tends to easily occur and the moisture resistance tends to fall.
The present invention can be applied to all multilayer piezoelectric devices comprised of piezoelectric layers and internal electrode layers alternately stacked, but the effect is particularly great when applied to a multilayer piezoelectric actuator device. Further, the effect is particularly large when the present invention is applied to a multilayer piezoelectric device where parts of the internal electrode layers are exposed at the surface of the device. In these cases, the moisture resistance has to be particularly superior.